Fuel Cell Separator and Method for Manufacturing the Same

ABSTRACT

There is provided a separator for a fuel cell having a very good anticorrosiveness and electrical conductivity. 
     A separator for a fuel cell includes: a base  1  formed of a steel which contains 10.5 mass % or more of Cr; a metal film  3  formed on the surface of the base  1 ; and an intermediate layer  2  formed between the base  1  and the metal film  3 , the intermediate layer  2  containing oxygen. The metal film  3  is composed of Au or Pt, and the intermediate layer  2  contains Fe and Cr which are contained in the steel and Au or Pt composing the metal film  3.

TECHNICAL FIELD

The present invention relates to a separator for a fuel cell, and inparticular to a separator which is suitable for a polymer electrolytefuel cell to be used in power sources for automobiles, power sources formobile devices, distributed generation sources, and the like.

BACKGROUND ART

Vigorous research activities are being directed to fuel cells as anext-generation energy source, due to their high generation efficiencyand low burden on the environment.

A fuel cell is a power generation device in which hydrogen as a fuel andoxygen are allowed to undergo an electrochemical reaction, from whichelectric energy is elicited. Depending on the type of electrolyte used,fuel cells are classified into: the Solid Oxide Fuel Cell (SOFC); theMolten Carbonate Fuel Cell (MCFC); the Phosphoric Acid Fuel Cell (PAFC);the Polymer Electrolyte Fuel Cell (PEFC); and the Direct Methanol FuelCell (DMFC). Among others, PEFCs and DMFCs have operating temperaturesas low as about 70 to 90° C. (as compared with other types of fuelcells), and enable a highly-efficient power generation even at about 1kW (PEFC) and about several hundred W (DMFC). Therefore, PEFCs and DMFCsare considered especially promising for applications such asautomobiles, mobile devices, and the like. In particular, DMFCs aresmall-sized, and their applications in mobile devices are beingvigorously studied.

Hereinafter, with reference to FIG. 3, the structure and principle of apolymer electrolyte fuel cell (PEFC) will be described as an example.

FIG. 3( a) is a perspective view schematically showing the structure ofa cell (battery) portion, which is a minimum structural unit of apolymer electrolyte fuel cell (PEFC). FIG. 3( b) is a schematic diagramshowing the principle behind a PEFC.

As shown in FIG. 3( a), a cell in a fuel cell includes an ion exchangemembrane (solid polymer membrane) 11 in the middle, with two electrodesbeing disposed on its sides: a fuel electrode (hydrogen electrode, anodeside) 12 and an air electrode (or oxygen electrode, cathode side) 13.The ion exchange membrane 11 is a membrane for allowing protons (H⁺) tomove from the fuel electrode 12 to the air electrode 13. It is often thecase that the ion exchange membrane 11 has electrode catalyst layers 14a and 14 b on both sides thereof, and thus the ion exchange membrane 11and the electrode catalyst layers 14 a and 14 b are collectivelyreferred to as a membrane electrode assembly (MEA) 20. On the outside ofthe fuel electrode 12 and the air electrode 13, separators 16 a and 16 bare provided via gaskets 15 a and 15 b, respectively. Thus, hydrogen(anode side) moves between the MEA 20 and the separator 16 a, and oxygen(cathode side) moves between the MEA 20 and the separator 16 b (see FIG.3( b)). On the surface of the separators 16 a and 16 b, trenches areformed through which reaction gases of hydrogen and oxygen pass.

As shown in FIG. 3( b), at the anode side, hydrogen (H₂) is suppliedthrough the trench in the surface of the separator 16 a, and uniformlydiffused into the electrode catalyst layer 14 a by the action of thefuel electrode 12. On the electrode catalyst layer 14 a, H₂ becomes H⁺through a reaction of formula (1) below, and passes through the ionexchange membrane 11 and moves to the electrode catalyst layer 14 b onthe cathode side. On the other hand, at the cathode side, oxygen (O₂) issupplied through the trench in the surface of the separator 16 b, anduniformly diffused into the electrode catalyst layer 14 b by the actionof the air electrode 13. On the electrode catalyst layer 14 b, areaction of formula (2) below occurs between the O₂ having been diffusedin this manner and the H⁺ which has moved through the ion exchangemembrane 11 from the anode side, whereby H₂O is generated.

H₂→2H+2e⁻  formula (1)

2H++½O₂+2e⁻→H₂O  formula (2)

At this time, power generation occurs due to the electrons (e⁻) whichare generated at the anode side. Therefore, the separator is required toefficiently supply reaction gases of oxygen and hydrogen to theelectrode catalyst layer 14 a.

Depending on the amount of electric power, a plurality of cells (unitcells) having the above construction may be layered so as to be used inthe form of a stack. In this case, the separators will act as partitionsbetween unit cells, and therefore are required to ensure that the gas(hydrogen) from the fuel electrode and the gas (oxygen) from the airelectrode will not become mixed in between cells.

From these standpoints, a separator is required to have little gaspermeability, a good electrical conductivity, a low contact resistance,a good anticorrosiveness, and so on. In particular, there emergestronger and stronger anticorrosiveness and electrical conductivityrequirements in the recent years. As an evaluation criterion ofanticorrosiveness, it is proposed that “no rust should occur even if theseparator is immersed for 1000 hours in a sulfuric acid solution whosepH is about 1”. In particular, DMFCs are small-sized and therefore arerequired to have a good surface electrical conductivity.

As a separator material having such characteristics, carbon materialsare generally used. However, carbon materials have poor toughness andare brittle, and therefore are difficult to process, thus resulting in aproblem of high processing costs.

Therefore, instead of carbon materials, use of metal materials asseparator materials has been proposed in the recent years, because metalmaterials are easy to process and incur low processing costs (PatentDocuments 1 to 2).

Above all, Patent Document 1 discloses a separator for a polymerelectrolyte fuel cell, in which an electrically conductive ceramiccoating such as niobium nitride, molybdenum silicide, tantalum carbide,or the like is provided on the surface of a stainless steel or the like.In Patent Document 1, in order to enhance the electrical conductivity inthe operating environment of a fuel cell (i.e., a temperature range fromroom temperature to near 150° C., vapor atmosphere), an electricallyconductive ceramic having a low resistivity is used.

Patent Document 2 discloses a separator for a polymer electrolyte fuelcell, in which a conductive film is formed on the surface of a metalsubstrate. The Cr concentration at the surface of the metal substrate isenhanced to no less than 13% or no less than 20%. At the interfacebetween the metal substrate and the conductive film, a Cr oxide layer isformed on the surface of the metal substrate, in order to enhance theanticorrosiveness in the operating environment of the battery (i.e.,about 80° C. in a saturated vapor).

However, the separators which are obtained by these methods still haveless than sufficient anticorrosiveness.

Other than the above, there have also been proposed separators in whicha plating of a metal film, e.g., a platinum group element or gold, isprovided on a stainless steel. Since an oxide coating (passivationcoating) is generated on the surface of the stainless steel, in whichthe Cr contained in the steel has bound to the oxygen in the atmosphere,a good anticorrosiveness is obtained but there is a large contactresistance. This is not usable as a separator material as it is.Accordingly, it might be conceivable to coat the surface of thestainless steel with a precious metal which excels is bothanticorrosiveness and electrical conductivity. However, adhesion betweena passivation coating and a metal film is very poor, and therefore it isvery difficult to directly form a metal film on the surface of astainless steel. Therefore, a method has hitherto been performed inwhich, after completely removing the passivation coating by etching orthe like, an underlying plating layer which contains a metal such as Niis formed as necessary, and then a plating of a precious metal isprovided.

[Patent Document 1] Japanese Laid-Open Patent Publication No. 11-162479

[Patent Document 2] Japanese Laid-Open Patent Publication No.2002-313355

[Patent Document 3] Japanese Laid-Open Patent Publication No. 10-68071

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, there is a problem in that, if a separator which is obtained bythe above method is used for a long time, it will deteriorate inanticorrosiveness, thus deteriorating the performance of the fuel cell.It is considered that this is because corrosive fluids will intrudethrough pinholes that are created in the metal film over long periods ofuse, thus allowing the surface of the stainless steel to be partiallyexposed. As a result, a local cell is created due to a potentialdifference between the stainless steel and the precious metal composingthe metal film, thus promoting bimetallic corrosion (galvaniccorrosion). Therefore, according to the above method, the aforementionedevaluation criterion for anticorrosiveness (“no rust should occur evenif the separator is immersed for 1000 hours in a strong acid solution”)cannot be satisfied.

Although separators for polymer electrolyte fuel cells have beenillustrated above for example, the required characteristics of aseparator, i.e., “good anticorrosiveness and electrical conductivity”are common not only to polymer electrolyte fuel cells, but also to otherfuel cells.

The present invention has been made in view of the above circumstances,and an objective thereof is to provide a separator for a fuel cellhaving a very good anticorrosiveness and electrical conductivity.

Means for Solving the Problems

A separator for a fuel cell according to the present inventioncomprises: a base formed of a steel which contains 10.5 mass % or moreof Cr; a metal film formed on a surface of the base; and an intermediatelayer formed between the base and the metal film, the intermediate layercontaining oxygen, wherein the metal film is composed of Au or Pt, andthe intermediate layer contains Fe and Cr which are contained in thesteel and Au or Pt composing the metal film.

In a preferred embodiment, a depth-direction profile showing an oxygencontent in the intermediate layer has a local maximum.

In a preferred embodiment, a depth-direction profile showing an Au or Ptcontent in the intermediate layer decreases from the metal film towardthe base and depth-direction profiles showing Fe and Cr contents in theintermediate layer increase from the metal film toward the base.

In a preferred embodiment, the steel further contains 5 to 16 mass % ofNi, and the intermediate layer contains: Fe, Cr, and Ni which arecontained in the steel; and Au or Pt composing the metal film.

In a preferred embodiment, a depth-direction profile showing an Au or Ptcontent in the intermediate layer decreases from the metal film towardthe base, and depth-direction profiles showing Fe, Cr, and Ni contentsin the intermediate layer increases from the metal film toward the base.

In a preferred embodiment, the intermediate layer has a thickness of0.050 μm or more when a thickness of the metal film is equal to orgreater than 0.03 μm and less than 0.10 μm; the intermediate layer has athickness of 0.030 μm or more when a thickness of the metal film isequal to or greater than 0.10 μm and less than 0.30 μm; and theintermediate layer has a thickness of 0.0020 μm or more when a thicknessof the metal film is equal to or greater than 0.30 μm.

In a preferred embodiment, the steel is a stainless steel.

In a preferred embodiment, the steel is an austenite-type stainlesssteel or an austenite-ferrite-type stainless steel.

In a preferred embodiment, the fuel cell is a polymer electrolyte fuelcell.

A fuel cell according to the present invention comprises theaforementioned separator for a fuel cell.

A method of producing a separator for a fuel cell according to thepresent invention comprises a step of providing a base formed of a steelwhich contains 10.5 mass % or more of Cr, with an oxide layer beingformed on at least a portion of a surface thereof, the oxide layercontaining oxides of Fe and Cr; a step of, through an ion bombardprocess, removing a portion of the oxide layer so as to leave a portionof the oxide layer; a step of forming an intermediate layer containingan element contained in the oxide layer and an element composing themetal film; and a step of, by a vapor deposition technique, depositing ametal film on the intermediate layer.

In a preferred embodiment, through the ion bombard process, a portion isremoved from the surface of the oxide layer to a depth of 0.0010 μm ormore.

In a preferred embodiment, the step of forming the intermediate layeralternately performs a step of depositing the metal film and a step ofperforming an ion bombardment process.

In a preferred embodiment, the vapor deposition technique is asputtering technique or an ion plating technique.

EFFECTS OF THE INVENTION

In accordance with a separator for a fuel cell according to the presentinvention, between a base formed of a steel which contains 10.5 mass %or more of Cr and a metal film, an intermediate layer containing oxygenis formed, the intermediate layer containing Fe and Cr (and additionallyNi) which are contained in the steel and Au or Pt composing the metalfilm, and therefore, a good anticorrosiveness and electricalconductivity is provided.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] A cross-sectional view schematically showing the constructionof a separator for a fuel cell according to the present invention.

[FIG. 2] Diagrams for describing element distributions in anintermediate layer along the depth direction, during a process offorming an intermediate layer in a production method for the separatoraccording to an embodiment of the present invention, where: (a) is agraph showing an element distribution along the depth direction in anFe/Cr/Ni oxide layer which is obtained through step 1 of theintermediate layer-forming process; (b) is a graph showing an elementdistribution along the depth direction in an Fe/Cr/Ni oxide layer whichis left from step 2; and (c) is a graph showing an element distributionalong the depth direction of the intermediate layer obtained throughstep 3.

[FIG. 3] (a) is a perspective view schematically showing the structureof a cell (battery) portion, which is a minimum structural unit of apolymer electrolyte fuel cell (PEFC); and (b) is a schematic diagramshowing the principle behind a PEFC.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   1 base    -   2 intermediate layer    -   3 metal film    -   11 ion exchange membrane (solid polymer membrane)    -   12 fuel electrode (hydrogen electrode)    -   13 air electrode (oxygen electrode)    -   14 a, 14 b electrode catalyst layer    -   15 a, 15 b gasket    -   16 a, 16 b separator

BEST MODE FOR CARRYING OUT THE INVENTION

With a view to providing a separator for a fuel cell which has a verygood anticorrosiveness and also a good electrical conductivity, theinventors have paid attention to a base composed of a stainless steelhaving a metal film coated thereon, and conducted continued studies tofurther enhance the anticorrosiveness of the base. As a result, theinventors have concluded that the conventional method which forms ametal film after completely removing a passivation coating that isformed on the surface of a stainless steel makes it difficult to providegood anticorrosiveness for a long period of time, and that it isessential to make use of the passivation coating in order to obtain adesired anticorrosiveness.

However, as described earlier, there is very poor adhesion between apassivation coating and a metal film. Therefore, the inventors haveconducted further studies concerning the method of depositing a metalfilm on a passivation coating. As a result, by adopting a method which“after using ion bombard process to remove a portion of a passivationcoating (Fe/Cr/Ni oxide layer) which is formed on the surface of astainless steel (i.e., a portion of the passivation coating remainsunremoved), alternately performs etching of the underlying layer anddeposition of a metal film”, the inventors have succeeded in forming,between the stainless steel and the metal film, an intermediate layerwhich contains elements that are contained in the passivation coating.Moreover, unlike the conventional passivation coating, this intermediatelayer contains not only elements that are contained in the passivationcoating but also an element composing the metal film, and thus has avery good adhesion to the metal film. Therefore, the separator of thepresent invention exhibits both a good anticorrosiveness due to thepassivation coating and a good anticorrosiveness and electricalconductivity due to Au or Pt, as well as a good adhesion between theintermediate layer and the metal film. Therefore, rust will not occureven if the separator of the present invention is immersed for 1000hours or more in a sulfuric acid solution whose pH is about 1, and avery good anticorrosiveness will be maintained for a long time.

Thus, in its technological concept of “utilizing the goodanticorrosiveness due to a passivation coating which is formed on thesurface of a stainless steel”, the present invention differs from theconventional technique which “completely removes the passivationcoating”. Conventionally, there has only been a concept that “since verypoor adhesion exists between a passivation coating and a metal film, thepassivation coating needs to be completely removed”, thus totallylacking in the inventive concept of “utilizing the goodanticorrosiveness due to the passivation coating”. Therefore, noattempts have been made to coat a metal film on the surface of astainless steel having a passivation coating formed thereon, whileleaving a portion of the passivation coating. Note that Patent Document1 discloses a metal coating comprising an electrically conductiveceramic dispersed on the surface of a stainless steel, and as metalscomposing the metal coating, exemplifies Au or Pt such as platinum groupelements or gold. However, after much scrutiny of Patent Document 1, nodisclosure is found of the present invention's technological concept of“enhancing anticorrosiveness by leaving a portion of the passivationcoating”, and not a single example of using Au or Pt is disclosed.Moreover, Patent Document 1 fails to disclose the unique inventivemethod of “after using ion bombard process to remove a portion of anFe/Cr/Ni oxide layer while leaving a portion thereof, alternatelyperforming etching of the underlying layer and deposition of a metalfilm”, and therefore does not provide any intermediate layer which“contains both elements composing the passivation coating and an elementcomposing the metal film” and which has good adhesion to the metal film.

Hereinafter, with reference to FIG. 1, an embodiment of the separatorfor a fuel cell according to the present invention will be described.FIG. 1 is a cross-sectional view schematically showing the constructionof a separator for a fuel cell according to the present embodiment. Asshown in FIG. 1, the separator for a fuel cell according to the presentembodiment includes a base 1 formed of a steel which contains 10.5 mass% or more of Cr; a metal film 3 formed on the surface of the base 1; andan intermediate layer 2 formed between the base 1 and the metal film 3.

The base 1 is formed of a steel which contains 10.5 mass % or more of Cr(chromium). As described above, the separator for a fuel cell accordingto the present invention utilizes the good anticorrosiveness due to apassivation coating composed of Cr oxide or the like which is formed onthe surface of a stainless steel. Therefore, a steel which contains atleast Cr as an anticorrosiveness element is used. Although theanticorrosiveness is more improved as the Cr content increases, anexcessive addition thereof will result in a reduced processibility.Therefore, it is preferable to keep the Cr content generally at 27 mass% or less. Preferably, the Cr content is no less than 13 mass % and nomore than 25 mass %.

In order to further enhance the anticorrosiveness, it is preferable that5 mass % or more of Ni (nickel) be contained. Although theanticorrosiveness is more improved as the Ni content increases, noimprovement in anticorrosiveness will be obtained even if the additionis made in excess of 16 mass %, and only a cost increase will result.Therefore, it is preferable to add it under an upper limit of generally16 mass %.

Other than that, in order to enhance characteristics such asanticorrosiveness and processibility, it is preferable that Cu (copper)be contained in a range of no more than about 3 mass % and that Mo(molybdenum silicide) be contained in a range of no more than about 5mass %.

Moreover, C (carbon) is to be added in order to secure a predeterminedstrength, but will deteriorate the processibility if added in largeamounts, and therefore is preferably kept under 1.0 mass % or less.

The steel to be used in the present embodiment preferably contains theaforementioned elements, and the remainder thereof is Fe (iron) andinevitable impurities.

As a steel that satisfies such requirements, it is preferable to use astainless steel.

Examples of stainless steels which do not contain (or substantially donot contain) Ni are ferrite-type stainless steels, martensite-typestainless steels, and the like. Ferrite-type stainless steels are steelswhich contain about 0.12 mass % or less of C and about 16 to 18 mass %of Cr, and representative examples thereof are 18 Cr-low carbonstainless steels such as SUS430 (JIS standard). Other examples are 18Cr-1 Mo stainless steels such as SUS434 (JIS standard), 18 Cr-2Mo—Ti/Nb/Zr-ultralow (C, N) stainless steels such as SUS444 (JISstandard), and the like. Martensite-type stainless steels are steelswhich contain about 0.1 to 0.4 mass % of C and about 12 to 18 mass % ofCr, and representative examples thereof are 13 Cr-low carbon stainlesssteels such as SUS410 (JIS standard), 18 Cr-high carbon stainless steelssuch as SUS440 (JIS standard), and the like.

Examples of stainless steels which contain Ni are austenite-typestainless steels, binary (austenite-ferrite) stainless steels, and thelike. Austenite-type stainless steels are steels which contain about0.15 mass % or less of C, about 16 to 20 mass % of Cr, and about 18 mass% or more of Ni, and representative examples thereof are 18 Cr-8 Ni-lowcarbon stainless steels such as SUS304, 18 Cr-12 Ni-2.5 Mo stainlesssteels such as SUS316, and the like. Examples of binary stainless steelsare 25 Cr-4.5 Ni-2 Mo stainless steels such as SUS329J1, and the like.Examples of martensite-type stainless steels containing Ni are 16 Cr-2Ni stainless steels such as SUS431, and the like.

Among the above, in terms of having a particularly goodanticorrosiveness, austenite-type stainless steels and binary stainlesssteel (which contain Ni within the steel) are preferable, theaustenite-type stainless steels being more preferable.

The intermediate layer 2 contains oxygen. More particularly, theintermediate layer 2 contains: Fe and Cr (in the case where the steelcontains Cr and contains substantially no Ni); or Fe, Cr and Ni (in thecase where the steel contains Cr and Ni), which are mainly contained inthe steel, and the intermediate layer 2 also contains Au or Pt composingthe metal film 3. Among these, it is considered that Fe and Cr (andadditionally Ni) bind to oxygen to mainly generate Fe oxide and Cr oxide(and additionally Ni oxide), respectively. A layer containing the aboveoxides corresponds to a passivation coating which is formed on thesurface of a stainless steel. In the present specification, within theintermediate layer 2, any layer that contains the above oxides may becollectively referred to as an “Fe/Cr/Ni oxide layer”.

As will be described later, in the separator of the present invention,before depositing the metal film 3, an ion bombard process is performedto remove a portion of an Fe/Cr/Ni oxide layer while leaving a portionthereof, and also a bias voltage or the like is varied within apredetermined range, whereby deposition of the metal film and removal ofthe underlying layer are alternately performed. Therefore, in theintermediate layer 2, not only the main elements composing the Fe/Cr/Nioxide layer but also Au or Pt (described later) that composes the metalfilm is deposited. The Au or Pt contained in the intermediate layerexists in the form of a metallic element, rather than as an oxide or thelike.

Depth-direction profiles showing the contents of the above elements inthe intermediate layer 2 are as follows. First, a depth-directionprofile showing the oxygen content has a local maximum, and decreasestoward the metal film 3 and toward the base 1. On the other hand,depth-direction profiles showing the Fe and Cr (and additionally Ni)contents increase from the metal film 3 toward the base 1, and adepth-direction profile showing the Au or Pt content decreases from themetal film 3 toward the base 1.

With reference to FIG. 2, element distributions in the intermediatelayer along the depth direction will be described. Herein, in order toexamine how elements composing the intermediate layer change with theprocess of forming the intermediate layer (steps 1 to 3 below), elementsin the layer obtained in each step were analyzed along the depthdirection.

First, a base formed of SUS304 steel was provided, and was heated at800° C. for 10 minutes in the atmosphere. As a result, an Fe/Cr/Ni oxidelayer having a thickness of 0.070 μm was formed on the surface of thebase (step 1).

Next, the base having the aforementioned oxide layer formed thereon wasplaced in an arc ion plating apparatus, which was evacuated untilreaching a vacuum of 2×10⁻³ Torr. Thereafter, an Ar gas was introduced,and by using Au as a target, an ion bombard process was performed whileapplying a bias voltage of −300 V, whereby a portion was removed, to adepth of 0.055 μm, from the surface of the Fe/Cr/Ni oxide layer. As aresult, an Fe/Cr/Ni oxide layer having a thickness of 0.015 μm was lefton the surface of the base (step 2).

Thereafter, the base was heated to 200° C., and Au was deposited whilesetting the bias voltage to −80 V and a portion of the underlying layerwas removed while setting the bias voltage to −180 V. Thus, depositionof the Au film and removal of the underlying layer were alternatelyperformed, thereby obtaining an intermediate layer (thickness: 0.028 μm)in which Au was deposited to a thickness of 0.06 μm (step 3).

The element distribution in each of the layers obtained from steps 1 to3 along the depth direction was measured by Electron Spectroscopy forChemical Analysis (ESCA). Specifically, “ESCA-850” manufactured byShimadzu Corporation was used, and measurements were taken whileirradiating each layer with MgK α (8.0 kV, 30 mA), which is thecharacteristic X ray of Mg (ion gun Ar, 2.0 kV, 20 mA).

These results are shown in FIGS. 2( a) to (c).

In each figure, the horizontal axis shows thickness (Å) along the depthdirection of each of the layers obtained from steps 1 to 3, whereas thevertical axis shows the Fe, Cr, Ni, and O (oxygen) contents (mass %)composing the layer. In FIGS. 2( a) and (b), a distance from the surface(horizontal axis=0) to a point where a depth-direction profile showingthe O content and a depth-direction profile showing the Fe contentintersect each other was defined as the “thickness of the Fe/Cr/Ni oxidelayer”. In FIG. 2( c), a distance from a point (near the surface) wherea depth-direction profile showing the O content intersects thehorizontal axis to a point where the depth-direction profile showing theO content intersects a depth-direction profile showing the Fe contentwas defined as the “thickness of the intermediate layer”. Moreover, adistance from the surface (horizontal axis=0) to a point (near thesurface) where the depth-direction profile showing the O contentintersects the horizontal axis was defined as the “thickness of the Aufilm”.

Note that a production method for the separator according to the presentinvention comprises, after step 3, a step of depositing an Au film to apredetermined thickness by vapor deposition technique. However, evenafter the addition of this step, the element distribution in theintermediate layer along the depth direction hardly changes from theelement distribution in the intermediate layer obtained from step 3.Therefore, the Au film depositing step is omitted herein.

FIG. 2( a) is a graph showing an element distribution along the depthdirection in the Fe/Cr/Ni oxide layer which is obtained through step 1.By heating, mainly oxides of Cr are formed near the surface of the oxidelayer.

FIG. 2( b) is a graph showing an element distribution along the depthdirection in the Fe/Cr/Ni oxide layer which is left from step 2. By anion bombard process, about ⅘ of the Fe/Cr/Ni oxide layer formed in step1 is removed, and oxides of Cr as well as oxides of Fe are formedabundantly near the surface of the remaining oxide layer.

FIG. 2( c) is a graph showing an element distribution along the depthdirection of the intermediate layer obtained through step 3. As shown inFIG. 2( c), the depth-direction profile showing the oxygen content has alocal maximum, and decreases toward the metal film 3 and toward the base1. On the other hand, the depth-direction profiles showing the Fe, Cr,and Ni contents and the depth-direction profile showing the Au contenthave opposite tendencies, such that the former increase from the metalfilm 3 toward the base 1, while the latter decreases from the metal film3 toward the base 1. Thus, by alternately performing deposition of theAu film and removal of the underlying layer while varying the biasvoltage, an intermediate layer is formed which contains elements thatare contained in the oxide layer as well as Au. Since such anintermediate layer has a good anticorrosiveness and also a good adhesionwith the subsequently-deposited Au film, a separator for a fuel cellincluding the intermediate layer has a very good anticorrosiveness.

The intermediate layer 2 of such a construction preferably has athickness of 0.050 μm or more when the thickness of the metal film 3 isequal to or greater than 0.03 μm and less than 0.10 μm, and preferablyhas a thickness of 0.030 μm or more when the thickness of the metal film3 is equal to or greater than 0.10 μm and less than 0.30 μm. Moreover,when the thickness of the metal film 3 is equal to or greater than 0.3μm, the intermediate layer 2 preferably has a thickness of 0.0020 μm ormore, and a sufficient anticorrosiveness will not be obtained if thethickness of the intermediate layer 2 is less than 0.0020 μm. Morepreferably, it is 0.010 μm or more, and further more preferably 0.020 μmor more. Although the anticorrosiveness is more improved as thethickness of the intermediate layer 2 increases, if it is too thick, along time will be required for forming the intermediate layer, which isunpractical. Therefore, the upper limit of the thickness of theintermediate layer may be determined from the standpoint ofproducibility rather than anticorrosiveness, and preferably has athickness of 0.1 μm or less, for example.

The metal film 3 is composed of Au or Pt. Since Pt is an expensivemetal, the most preferable precious metal is Au, in terms of cost.

The metal film 3 preferably has a thickness of 0.03 μm or more. If thethickness of the metal film 3 is less than 0.03 μm, the desiredanticorrosiveness cannot be obtained. More preferably, it is 0.05 μm ormore, and further more preferably 0.10 μm or more. Although theanticorrosiveness is more improved as the thickness of the metal film 3increases, if it is too thick, a long time will be required for formingthe metal film 3, which is unpractical. Therefore, the upper limit ofthe thickness of the metal film 3 may be determined from the standpointof producibility rather than anticorrosiveness, and preferably has athickness of 1.5 μm or less, for example. In order to obtain a goodanticorrosiveness when used as a separator, it is preferable that atleast the portion which is in contact with the electrolyte is coatedwith the metal film 3.

According to the production method of the present invention, avapor-deposited film 3 of a precious metal which has a very goodadhesion with the intermediate layer 2 is obtained.

Next, a method for producing the separator for a fuel cell according tothe present invention will be described.

The production method of the present invention comprises: a step ofproviding a base such that an Fe/Cr/Ni oxide layer is formed on at leasta portion of the surface of the base, the base being formed of a steelwhich contains 10.5 mass % or more of Cr (and which may additionallycontain Ni; hereinafter may be referred to as a “Cr/Ni-containingsteel”); a step of removing a portion of the Fe/Cr/Ni oxide layer so asto leave a portion of the oxide layer by an ion bombard process; a stepof forming an intermediate layer containing elements contained in theoxide layer and an element contained in the metal film; and a step ofdepositing a metal film on the intermediate layer by vapor depositiontechnique.

Before describing the respective steps in detail, the “vapor depositiontechnique” will first be described.

In the present specification, a “vapor deposition technique” broadlyencompasses any method that allows the substance-to-be-deposited (i.e.,a precious metal in the present invention) to be deposited in a gaseousstate, and includes physical vapor deposition technique and CVD(chemical vapor deposition) technique. In the present invention, it ismore preferable to use physical vapor deposition technique as the vapordeposition technique. In particular, since films that are formed by ionplating technique and sputtering technique are generally dense, themetal film is more preferably formed by ion plating technique orsputtering technique.

Examples of ion plating technique include activated reactive evaporationtechnique, high-frequency excitation technique, hollow cathode dischargetechnique, arc evaporation technique (arc ion plating technique), andthe like. In the case of using sputtering technique, a DC sputteringapparatus, an RF sputtering apparatus, a magnetron sputtering apparatus,an ion beam sputtering apparatus or the like may be used.

First, a base is provided such that an Fe/Cr/Ni oxide layer is formed onat least a portion of the surface of the base, the based being formed ofa Cr/Ni-containing steel. Such a base may be a commercially-availableproduct, or a base which is formed of a Cr/Ni-containing steel may beheated to form an Fe/Cr/Ni oxide layer.

A preferable thickness of the Fe/Cr/Ni oxide layer is about 0.0030 to0.20 μm. Since a portion (about 0.0010 μm or more) of the oxide layer isto be subsequently removed by an ion bombard process, it is preferablethat the oxide layer is slightly thicker than the thickness (about0.0020 to 0.1 μm) of the intermediate layer that is finally obtained. Asthe specific heating conditions, appropriate conditions may be set sothat a desired thickness is obtained, in accordance with the type ofbase used or the type of precious metal composing the metal film, theconditions of the ion bombard process, and the like. However, generallyspeaking, a heating at 500 to 800° C. for 0.02 to 0.5 hours ispreferable. The aforementioned heat treatment is preferably performedwithin a vapor deposition apparatus which is used for the subsequent ionbombard process and metal film depositing process, whereby the series ofprocesses can be efficiently performed.

Next, an ion bombard process is performed for the Fe/Cr/Ni oxide layer.In the present invention, it is necessary to perform the ion bombardprocess so that a portion of the Fe/Cr/Ni oxide layer is removed but aportion thereof remains. As a result, the good anticorrosiveness due tothe Fe/Cr/Ni oxide layer can be effectively exhibited.

The ion bombard process (metal ion bombard process) is a kind of removalof a coating (etching), which is performed to obtain a thin film havinga good adhesion, and is performed by allowing metal ions (i.e., preciousmetal ions in the present embodiment) to collide with a material whichis subjected to deposition. By performing an ion bombard process, anmetal film can be deposited on the Fe/Cr/Ni oxide layer with a goodadhesion.

Specifically, the ion bombard process is performed as follows. First, anAr gas is introduced into a vacuum container which is adjusted to arange of about 1 to 3×10⁻³ Torr (about 133 to 399 Pa), and whileapplying a negative bias voltage (about −150 to −180 V or even greater)to the material which is subjected to deposition (i.e., the base withthe Fe/Cr/Ni oxide layer formed thereon), cations of a precious metalare generated through evaporation from a precious metal target(cathode). Due to the negative bias voltage which is applied to thematerial subjected to deposition, the ions will collide with thematerial subjected to deposition, whereby the surface of the materialsubjected to deposition is sputtered. Herein, it is preferable that aportion to a depth of about 0.0010 μm is finally removed from thesurface of the Fe/Cr/Ni oxide layer, while paying particular attentionto the bias voltage which is applied to the substrate. The reason isthat the bias voltage is considered to greatly affect the speed ofremoval (etching rate) of the Fe/Cr/Ni oxide layer, the surfaceattributes after the ion bombard process, and the like, thus affectingthe adhesion between the Fe/Cr/Ni oxide layer and the metal film. Theoxide layer cannot be efficiently removed if the bias voltage is lessthan about −150 V. There is no particular upper limit to the biasvoltage, which may be appropriately set in accordance with the apparatusused.

Note that a general outline of the ion bombard process is specificallydescribed in Patent Document 3, for example, and thus Patent Document 3and the like may be referred to for details of the apparatus and thelike. The entire disclosure of Patent Document 3 is incorporated hereinby reference.

Next, an intermediate layer which contains elements contained in theFe/Cr/Ni oxide layer and an element composing the metal film is formed.In the present embodiment, by using the same apparatus as that used forthe above-described ion bombard process, the intermediate layer isformed by alternately performing a step of depositing the metal film anda step of etching the underlying layer through an ion bombardmentprocess. Specifically, the intermediate layer is preferably formed byvarying the bias voltage as follows. Preferably, Au or Pt is firstdeposited while applying a bias voltage in a range of about −75 to −85Vfor about 10 to 20 seconds, and thereafter a portion of the underlyinglayer (including a portion of the Au or Pt which has been deposited inthe above manner) is removed while applying for about 5 to 15 seconds abias voltage which is controlled to a range of about −150 V or greater.Such a series of operations is preferably repeated a plurality of times(about 3 to 10 times), whereby a desired thickness of the intermediatelayer can be obtained. When the above operations are to be repeated aplurality of times, the bias voltage when etching a portion of theunderlying layer may be changed each time within the aforementionedrange. For example, the bias voltage may be sequentially decreased,e.g., about −170 V for the first etching, −160 V for the second etching,−150 V for the third etching, and so on.

In order to efficiently perform the above operations, it is preferableto pre-heat the material which is subjected to deposition to atemperature of about 150 to 250%.

Finally, on the intermediate layer which has been formed in the abovemanner, a metal film is deposited by vapor deposition technique.

The specific film-forming conditions may vary depending on the type ofvapor deposition technique used. However, a method of depositing a metalfilm by using arc ion plating technique may be as follows, for example.

First, within a vacuum container which is adjusted to a range of about 2to 5×10⁻³ Torr (about 266 to 665 Pa), a reaction gas such as a nitrogengas, an oxygen gas, or an inert gas is introduced, and a negative biasvoltage is applied to the material subjected to deposition (i.e., theaforementioned intermediate layer), thus performing an arc discharge toevaporate the cathode material. The bias voltage to be applied to thematerial subjected to deposition is preferably smaller than whenperforming the ion bombard process, whereby a metal film is deposited onthe surface of the material subjected to deposition. The bias voltage tobe applied is preferably −60 to −70 V to permit efficient deposition ofthe metal film. The metal film cannot be efficiently deposited if thebias voltage is less than −60 V. On the other hand, if the bias voltageexceeds −70 V, the surface roughness will become very large, thusinviting surface defects such as pinholes.

The separator for a fuel cell according to the present invention, whichhas been produced in this manner, is particularly suitably used for apolymer electrolyte fuel cell.

EXAMPLES

In the following Experimental Examples, with respect to materials inwhich the surface of a base formed of a stainless steel was coated withvarious intermediate layers and metal films, anticorrosiveness andelectrical conductivity were evaluated (Experimental Examples 1 to 3).For comparison, conventional examples (Comparative Examples 1 to 3) inwhich the oxide layer formed on the surface of a base was completelyremoved, and comparative examples (Comparative Examples 4 to 5) in whichthe oxide layer and metal film were reduced in thickness were produced,and their anticorrosiveness and electrical conductivity were evaluatedsimilarly to the Experimental Examples above (Comparative Examples 1 to5).

The details of the evaluation method are as follows.

[Anticorrosiveness]

Each material was immersed in a sulfuric acid solution (35° C.) at pH1.The rust generation in 1000 hours and 1500 hours after immersion wasobserved with a stereomicroscope (magnification×30). In the presentspecification, “good anticorrosiveness” is characterized by the absenceof rust after 1000 hours of immersion in a sulfuric acid solution at pH1.

[Electrical Conductivity]

By using “196 System DMM” manufactured by KEITHLEY Instruments, Inc.,the surface resistance of each material was measured.

In the present specification, “good electrical conductivity” ischaracterized by the surface resistance of a material as measured by theabove method satisfying the range of 0.01 to 0.03Ω.

Note that the thicknesses of the intermediate layer and metal filmformed on the surface of the base were both measured by theaforementioned ESCA technique.

Experimental Example 1

In this Experimental Example, a material was produced such that thesurface of a base formed of a stainless steel was coated with anintermediate layer containing elements composing the Fe/Cr/Ni oxidelayer and Au (thickness: 0.015 μm) and with an Au film (thickness: 0.8μm).

First, a base formed of SUS304 steel (80 mm long×80 mm wide×1.0 mmthick) was provided, and heated at 800° C., for 3 minutes in theatmosphere. As a result, on the surface of the base, an Fe/Cr/Ni oxidelayer having a thickness of 0.020 μm was formed.

Next, the base having the oxide layer formed on its surface was placedin an arc ion plating apparatus, which was evacuated to a vacuum. An Argas was introduced, and at a gas pressure of 2×10⁻³ Torr (about 266×10⁻³Pa), by using Au as a target, an ion bombard process was performed whileapplying a bias voltage of −300 V. Thus, a portion was removed, to adepth of 0.010 μm, from the surface of the Fe/Cr/Ni oxide layer. As aresult, an Fe/Cr/Ni oxide layer having a thickness of 0.010 μm was lefton the surface of the base.

Thereafter, the base was heated to 200° C., and Au was deposited through15 seconds of application of a bias voltage which was set to −80 V, anda portion of the underlying layer was removed through 5 seconds ofapplication of a bias voltage which was set to −180 V. Thus, depositionof the Au film and removal of the underlying layer were alternatelyperformed. Such a series of operations was carried out six times. As aresult, an intermediate layer in which Au was deposited to a thicknessof 0.05 μm was obtained (thickness of the intermediate layer: 0.015 μm).

Finally, while applying a bias voltage of −70 V, an Au film having athickness of 0.8 μm was deposited.

Experimental Example 2

In this Experimental Example, the method of Experimental Example 1 wasmodified so as to produce a material such that the surface of a baseformed of a stainless steel was coated with an intermediate layercontaining elements composing the Fe/Cr/Ni oxide layer and Au(thickness: 0.008 μm) and with an Au film (thickness: 0.5 μm).

First, except that the heating time was reduced to 1.5 minutes, aheating was performed similarly to Experimental Example 1, therebyforming an Fe/Cr/Ni oxide layer having a thickness of 0.010 μm on thesurface of a base formed of SUS304 steel.

Next, an ion bombard process was performed similarly to ExperimentalExample 1, thereby removing a portion, to a depth of 0.005 μm, from thesurface of the Fe/Cr/Ni oxide layer. Therefore, an Fe/Cr/Ni oxide layerhaving a thickness of 0.005 μm was left on the surface of the base.

Thereafter, the bias voltage was varied similarly to ExperimentalExample 1, whereby an intermediate layer in which 0.03 μm of an Au filmwas deposited was obtained (thickness of the intermediate layer: 0.008μm).

Finally, while applying a bias voltage of −65V, an Au film having athickness of 0.5 μm was deposited.

Experimental Example 3

In this Experimental Example, the method of Experimental Example 1 wasmodified so as to produce a material such that the surface of a baseformed of a stainless steel was coated with an intermediate layercontaining elements composing the Fe/Cr/Ni oxide layer and Pt (0.015 μm)and with a Pt film (1.2 μm).

First, except that the heating time was prolonged to 4 minutes, aheating was performed similarly to Experimental Example 1, therebyforming an Fe/Cr/Ni oxide layer having a thickness of 0.025 μm on thesurface of a base formed of SUS304 steel.

Next, except that a Pt target was used, an ion bombard process wasperformed similarly to Experimental Example 1, thereby removing aportion, to a depth of 0.010 μm, from the surface of the Fe/Cr/Ni oxidelayer. Therefore, an Fe/Cr/Ni oxide layer having a thickness of 0.015 μmwas left on the surface of the base.

Thereafter, the bias voltage was varied similarly to ExperimentalExample 1, whereby an intermediate layer in which a Pt film having athickness of 0.05 μm was deposited was obtained (thickness of theintermediate layer: 0.020 μm).

Finally, while applying a bias voltage of −70V, a Pt film having athickness of 1.2 μm was deposited.

Experimental Example 4

In this Experimental Example, the method of Experimental Example 1 wasmodified so as to produce a material such that the surface of a baseformed of a stainless steel was coated with an intermediate layercontaining elements composing the Fe/Cr/Ni oxide layer and Au(thickness: 0.060 μm) and with an Au film (thickness: 0.050 μm).

First, except that the heating time was reduced to 1.5 minutes, aheating was performed similarly to Experimental Example 1, therebyforming an Fe/Cr/Ni oxide layer having a thickness of 0.070 μm on thesurface of a base formed of SUS304 steel.

Next, an ion bombard process was performed similarly to ExperimentalExample 1, thereby removing a portion, to a depth of 0.010 μm, from thesurface of the Fe/Cr/Ni oxide layer. Therefore, an Fe/Cr/Ni oxide layerhaving a thickness of 0.060 μm was left on the surface of the base.

Thereafter, the bias voltage was varied similarly to ExperimentalExample 1, whereby an intermediate layer in which 0.010 μm of an Au filmwas deposited was obtained (thickness of the intermediate layer: 0.065μm).

Finally, while applying a bias voltage of −65V, an Au film having athickness of 0.050 μm was deposited.

Experimental Example 5

In this Experimental Example, the method of Experimental Example 1 wasmodified so as to produce a material such that the surface of a baseformed of a stainless steel was coated with an intermediate layercontaining elements composing the Fe/Cr/Ni oxide layer and Au (0.035 μm)and with an Au film (0.12 μm).

First, except that the heating time was prolonged to 4 minutes, aheating was performed similarly to Experimental Example 1, therebyforming an Fe/Cr/Ni oxide layer having a thickness of 0.040 μm on thesurface of a base formed of SUS304 steel.

Next, except that an Au target was used, an ion bombard process wasperformed similarly to Experimental Example 1, thereby removing aportion, to a depth of 0.010 μm, from the surface of the Fe/Cr/Ni oxidelayer. Therefore, an Fe/Cr/Ni oxide layer having a thickness of 0.030 μmwas left on the surface of the base.

Thereafter, the bias voltage was varied similarly to ExperimentalExample 1, whereby an intermediate layer in which an Au film having athickness of 0.05 μm was deposited was obtained (thickness of theintermediate layer: 0.035 μm).

Finally, while applying a bias voltage of −70V, a Pt film having athickness of 0.12 μm was deposited.

Comparative Example 1

Except that a bias voltage of −400 V was applied and the Fe/Cr/Ni oxidelayer was completely removed in the ion bombard process of ExperimentalExample 1, a similar processing to that of Experimental Example 1 wasperformed, thereby producing a material such that the surface of a baseformed of a stainless steel was coated with an Au film (thickness: 0.8μm).

Comparative Example 2

Except that a bias voltage of −400 V was applied and the Fe/Cr/Ni oxidelayer was completely removed in the ion bombard process of ExperimentalExample 2, a similar processing to that of Experimental Example 2 wasperformed, thereby producing a material such that the surface of a baseformed of a stainless steel was coated with an Au film (thickness: 0.5μm).

Comparative Example 3

Except that a bias voltage of −400 V was applied and the Fe/Cr/Ni oxidelayer was completely removed in the ion bombard process of ExperimentalExample 3, a similar processing to that of Experimental Example 3 wasperformed, thereby producing a material such that the surface of a baseformed of a stainless steel was coated with a Pt film (thickness: 1.2μm).

Comparative Example 4

Except that a bias voltage of −320 V was applied and a portion of theFe/Cr/Ni oxide layer was allowed to remain (thickness: 0.0011 μm) in theion bombard process of Experimental Example 1, a similar processing tothat of Experimental Example 1 was performed, thereby producing amaterial such that the surface of a stainless steel was coated with anintermediate layer containing elements composing the Fe/Cr/Ni oxidelayer and Au (thickness: 0.0016 μm) and with an Au film (thickness: 0.8μm).

Comparative Example 5

Except that the deposition time was reduced from 0.36 hours to 0.12hours and an Au film having a thickness of 0.27 μm was deposited inExperimental Example 1, a similar processing to that of ExperimentalExample 1 was performed, thereby producing a material such that thesurface of a base formed of a stainless steel was coated with anintermediate layer containing elements composing the Fe/Cr/Ni oxidelayer and Au (thickness: 0.010 μm) and with an Au film (thickness: 0.27μm).

Comparative Example 6

Except that a bias voltage of −320 V was applied and a portion of theFe/Cr/Ni oxide layer was allowed to remain (thickness: 0.025 μm) in theion bombard process of Experimental Example 5, a similar processing tothat of Experimental Example 5 was performed, thereby producing amaterial such that the surface of a stainless steel was coated with anintermediate layer containing elements composing the Fe/Cr/Ni oxidelayer and Au (thickness: 0.030 μm) and with an Au film (thickness: 0.08μm).

Comparative Example 7

Except that the deposition time was reduced and an Au film having athickness of 0.020 μm was deposited in Experimental Example 4, a similarprocessing to that of Experimental Example 4 was performed, therebyproducing a material such that the surface of a base formed of astainless steel was coated with an intermediate layer containingelements composing the Fe/Cr/Ni oxide layer and Au (thickness: 0.065 μm)and with an Au film (thickness: 0.020 μm).

Results of anticorrosiveness and electrical conductivity for eachmaterial are summarized in Table 1.

TABLE 1 electrical anticorrosiveness conductivity 1000 hours 1500 hours(Ω) Experimental No rust occurs No rust occurs 0.01 to 0.02 Example 1Experimental No rust occurs Rust spots 0.01 to 0.02 Example 2 (3 ormore) Experimental No rust occurs No rust occurs 0.02 to 0.03 Example 3Experimental No rust occurs No rust occurs 0.01 to 0.02 Example 4Experimental No rust occurs No rust occurs 0.01 to 0.02 Example 5Comparative Rust spots Portion of 0.05 to 0.06 Example 1 (20 or more) Aufilm peels Rust occurs in steel Comparative Portion of Large rust 0.05to 0.06 Example 2 Au coating occurs in peels steel Large rust occurs insteel Comparative Rust spots Portion of 0.05 to 0.06 Example 3 (10 ormore) Pt film peels Comparative Rust spots Portion of 0.01 to 0.02Example 4 (15) Au coating peels Rust occurs in steel Comparative Rustspots Increased rust 0.05 to 0.06 Example 5 (5) spots (20 or more)Comparative Rust spots Increased rust 0.04 to 0.06 Example 6 (3) spots(10 or more) Comparative No rust occurs A few rust 0.05 to 0.06 Example7 spots

As shown in Table 1, all of the materials of Experimental Examples 1 to3, in which the surface of a base formed of a stainless steel is coatedwith a predetermined intermediate layer and metal film of appropriatethicknesses, have a good anticorrosiveness and electrical conductivity.

On the other hand, in the materials of Comparative Examples 1 to 3, inwhich the Fe/Cr/Ni oxide layer is completely removed, rust spotsoccurred and/or the metal film peeled.

Moreover, in both Comparative Example 4 (in which the thickness of theintermediate layer is below the preferable range) and ComparativeExample 5 (in which the thickness of the metal film is below thepreferable range), rust was observed in 1000 hours after immersion.Furthermore, electrical conductivity deteriorated in Comparative Example5.

Although the metal film was deposited by using an arc ion platingtreatment in the above Experimental Examples, this is no limitation. Ithas been experimentally confirmed that similar results are also obtainedwith other vapor deposition techniques.

INDUSTRIAL APPLICABILITY

A separator for a fuel cell according to the present invention has agood anticorrosiveness and electrical conductivity, and therefore can bebroadly used for power sources for automobiles, power sources for mobiledevices, distributed generation sources, and the like.

1. A separator for a fuel cell, comprising: a base formed of a steelwhich contains 10.5 mass % or more of Cr; a metal film formed on asurface of the base; and an intermediate layer formed between the baseand the metal film, the intermediate layer containing oxygen, whereinthe metal film is composed of Au or Pt, and the intermediate layercontains Fe and Cr which are contained in the steel and Au or Ptcomposing the metal film.
 2. The separator for a fuel cell of claim 1,wherein a depth-direction profile showing an oxygen content in theintermediate layer has a local maximum.
 3. The separator for a fuel cellof claim 1, wherein a depth-direction profile showing an Au or Ptcontent in the intermediate layer decreases from the metal film towardthe base and depth-direction profiles showing Fe and Cr contents in theintermediate layer increase from the metal film toward the base.
 4. Theseparator for a fuel cell of claim 1, wherein the steel further contains5 to 16 mass % of Ni, and the intermediate layer contains: Fe, Cr, andNi which are contained in the steel; and Au or Pt composing the metalfilm.
 5. The separator for a fuel cell of claim 4, wherein adepth-direction profile showing an Au or Pt content in the intermediatelayer decreases from the metal film toward the base, and depth-directionprofiles showing Fe, Cr, and Ni contents in the intermediate layerincreases from the metal film toward the base.
 6. The separator for afuel cell of claim 1, wherein the intermediate layer has a thickness of0.050 μm or more when a thickness of the metal film is equal to orgreater than 0.03 μm and less than 0.10 μm; the intermediate layer has athickness of 0.030 μm or more when a thickness of the metal film isequal to or greater than 0.10 μm and less than 0.30 μm; and theintermediate layer has a thickness of 0.0020 μm or more when a thicknessof the metal film is equal to or greater than 0.30 μm.
 7. The separatorfor a fuel cell of claim 1, wherein the steel is a stainless steel. 8.The separator for a fuel cell of claim 4, wherein the steel is anaustenite-type stainless steel or an austenite-ferrite-type stainlesssteel.
 9. The separator for a fuel cell of claim 1, wherein the fuelcell is a polymer electrolyte fuel cell.
 10. A fuel cell comprising anyone of the separators for a fuel cell of claim
 1. 11. A method ofproducing a separator for a fuel cell, comprising: a step of providing abase formed of a steel which contains 10.5 mass % or more of Cr, with anoxide layer being formed on at least a portion of a surface thereof theoxide layer containing oxides of Fe and Cr; a step of, through an ionbombard process, removing a portion of the oxide layer so as to leave aportion of the oxide layer; a step of forming an intermediate layercontaining an element contained in the oxide layer and an elementcomposing the metal film; and a step of, by a vapor depositiontechnique, depositing a metal film on the intermediate layer.
 12. Themethod of producing a separator for a fuel cell of claim 11, wherein,through the ion bombard process, a portion is removed from the surfaceof the oxide layer to a depth of 0.0010 μm or more.
 13. The method ofproducing a separator for a fuel cell of claim 11, wherein the step offorming the intermediate layer alternately performs a step of depositingthe metal film and a step of performing an ion bombardment process. 14.The method of producing a separator for a fuel cell of claim 11, whereinthe vapor deposition technique is a sputtering technique or an ionplating technique.